--- a/hotspot/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.cpp	Fri Aug 31 16:39:35 2012 -0700
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.cpp	Sat Sep 01 13:25:18 2012 -0400
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2011, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2012, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -382,8 +382,16 @@
   // If the fist object's end q is at the card boundary. Start refining
   // with the corresponding card (the value of the entry will be basically
   // set to 0). If the object crosses the boundary -- start from the next card.
+  size_t n_index = _array->index_for(n);
   size_t next_index = _array->index_for(n) + !_array->is_card_boundary(n);
-  HeapWord* next_boundary = _array->address_for_index(next_index);
+  // Calculate a consistent next boundary.  If "n" is not at the boundary
+  // already, step to the boundary.
+  HeapWord* next_boundary = _array->address_for_index(n_index) +
+                            (n_index == next_index ? 0 : N_words);
+  assert(next_boundary <= _array->_end,
+         err_msg("next_boundary is beyond the end of the covered region "
+                 " next_boundary " PTR_FORMAT " _array->_end " PTR_FORMAT,
+                 next_boundary, _array->_end));
   if (csp() != NULL) {
     if (addr >= csp()->top()) return csp()->top();
     while (next_boundary < addr) {